The term “hermetic” means a seal that is gas tight or impervious to gas flow. In the context of microelectronics it implies an airtight seal that will keep moisture and other harmful gases from penetrating the sealed package. Metals, ceramics and glasses are the materials used to form hermetic packages and prevent water vapor from contaminating components inside the package. A properly made hermetic seal with a sufficiently low leak rate can keep a package dry and moisture free for many years.
Mil-STD-883 Test Method 1014 is the universally accepted test designed to determine the effectiveness or hermeticity of the seal. There are several techniques for testing hermeticity, but the most common method is to measure the rate at which helium escapes from a package that has been pressurized or backfilled with helium (the tracer gas). This measured helium leak rate is then correlated with an “air” leak rate. The hermeticity specification is based on a maximum “air” leak rate for a given package volume. The exact definition of hermeticity is defined in Test Method 1014 paragraph 3.1.1.2.1, which lists the failure criteria for a given package volume in terms of an air equivalent leak rate. When a cavity-sealed microelectronic package passes both gross and fine leak test per Test Method 1014 the part is deemed “hermetic”.
Polymeric materials such as silicones and epoxies do not provide a hermetic seal and cannot be used to improve or fix a hermetic seal. Cavity packages made from polymers (e.g. LCP) or molded/potted microelectronics are known in the industry as “near-hermetic” or “non- hermetic”. A “near” or “non” hermetic configuration provide enhanced resistance to moisture entry into a package, but they are not hermetic as defined by the military standards. If liquid droplets form on the surface of an IC or other active devices sensitive to moisture, then corrosion or other electrochemical reactions may occur and degrade the performance of the device and ultimately lead to failure. Moisture droplets can form as the package is cooled below the dew point. This surface water (H2O) combines with any available surface ionic contamination, particularly sodium (Na) or chlorine (Cl), and along with a bias will chemically attack and corrode exposed aluminum metal at wirebond pads. The conductor metallization beneath defects, such as, cracks or pinholes in device passivation, or thin film resistor networks are also susceptible to this type of failure mechanism. Three monolayers of moisture on the surface is all that is needed to sustain surface conduction and facilitate electrochemical reactions.
Other problems caused by moisture inside a package include: electrical leakage across pins, damage to the doped layers on a silicon chip if there are pinholes in the surface passivation, arcing in a high voltage device, fogging of optical components, and “stiction” of moving parts in microelectromechanical (MEMS) components. Moisture related problems over the years have been well chronicled in technical journals and discussed at length at conferences.